One objective of this project is to obtain information about the organization of tRNA genes in Drosophila melanogaster. tRNAs will be purified, radiolabeled and localized on the polytene chromosome by in situ hybridization. The pure tRNAs will also be used to select clones of E. coli with plasmids containing Drosophila DNA fragments that have tRNA genes. The tRNA genes will be located on the restriction map of the DNA fragments by hybridization. The knowledge of tRNA gene locations will be used for the two other major objectives of this project. First, the technique of in situ hybridization will be extended so it can be used reliably to localize sequences complimentary to the cloned DNA on diploid metaphase chromosomes. The availability of tRNA gene plasmids which will localize all over the genetic map and polytene chromosomes to check the localization make this an ideal model system. Second, the expression of localized tRNA genes will be examined in various strains of Drosophila with mutations at that locus. The extent of expression will be measured by the level of aminoacylation of the given tRNA.